1. Field of the Invention
The present invention relates generally to a multi-chip type semiconductor device, and more particularly to a lead frame for mounting semiconductor chips to provide a multi-pin QFP (Quad Flat Package).
2. Description of the Related Art
With the development in integrated circuits having a high density and advanced functions, a conventional one-chip package system has now been insufficient to meet the present requirements. Under the situation, a multi-chip package system including a plurality of semiconductor chips in one package has been developed.
By virtue of a multi-chip package, high-density packaging is realized. Accordingly, for developing new integrated circuits, only a lead frame need be designed since existing chips are combined to assemble them in one package. Thus, the time for development can be reduced. Such devices as silicon ICs and GaAs devices, or bipolar and C-MOS ICs, which have not easily been assembled in one chip in the prior art, can be incorporated in one chip, resulting in advancement of functions. Further, since these devices are molded be transfer molding, the similar reliability to that of a conventional resin-molded type semiconductor device can be obtained. The usefulness of this type of package is remarkable.
Resin-molded type packages of semiconductor devices such as ICs and LSIs have recently become popular more and more, with a view to reducing the cost and in accordance with the complicated configuration. For providing a multi-chip type package for this type semiconductor device, it is necessary to mount a plurality of chips on a lead frame and to mold them with a resin. FIG. 4 shows an example of a lead frame on which two chips are mounted. The lead frame 1 has rectangular islands 2 at a center to mount rectangular chips thereon. A desired one of the sides of each island 2 is parallel to the corresponding side of the lead frame 1. The islands 2 are supported by island supports 3. Leads are also supported by tie bars 4. After the chips are mounted on the islands 2, the chips, island support 3 and leads are covered with a mold resin such as epoxy resins. Outer leads of the leads, which extend from the tie bars 4, are exposed, but the chips, bonding wires electrically connected to the leads, inner leads 5, and intermediate leads between the inner leads and outer leads are molded with the resin. Injection molding, transfer molding and dip molding have been known as resin molding techniques. However, transfer molding has been most widely employed.
The number of pins of a multi-chip package has been increasing year after year as electronic devices have the higher density and highly advanced functions. In the case where the number of pins (outer leads) is not so much as shown in FIG. 4, there is no problem. However, when two chips are provided in a package 6 as shown in FIG. 5, a large space may be required for leading intermediate leads located between the two chips. As a result, the package is inevitably elongated, and it is impossible to accommodate the chips in the existing package. If this is forcibly applied to the existing package, the width of each intermediate lead must be reduced, and the leads must be arranged in such a manner that the intermediate leads are greatly bent. Consequently, the strength of leads may be reduced to beak them easily due to shock caused during transfer of the chips after finishing wire bonding and to deform them during resin molding. Thus, problems associated with assembly and reliability may occur.
In addition, in many cases, since the multi-chip packages are combined with existing semiconductor chips, a plurality of electrodes on the chip, i.e. bonding pads, may possibly be arranged non-uniformly. In particular, in chips of bipolar ICs, bonding pads are often arranged non-uniformly, because of the nature of the circuits. When the bonding pads are arranged nonuniformly on the chip, unoccupied space and useless leads 8 may be provided as shown in FIG. 6.
As described above, in the conventional packages, it is difficult to effectively arrange chips having many pins and irregularly arranged boading pads. Thus, the size of the packages may be undesirably increased.